[matter] add support for --sync-best-effort

matter/matter/main.py

@@ -55,7 +55,7 @@ def matter_main(binary_pms, nsargs, cwd, specs):
     # sync portage
     if nsargs.sync:
         _rc = PackageBuilder.sync()
-        if _rc != 0:
+        if _rc != 0 and not nsargs.sync_best_effort:
             raise SystemExit(_teardown(_rc))
 
     exit_st = 0
@@ -302,6 +302,12 @@ Available Binary PMSs:
         help="sync Portage tree, and attached overlays, before starting.",
         action="store_true")
 
+    parser.add_argument(
+        "--sync-best-effort", default=False,
+        help="sync Portage tree and attached overlays, as --sync, but do "
+        "not exit if sync fails.",
+        action="store_true")
+
     parser.add_argument(
         "--disable-preserved-libs",
         dest="disable_preserved_libs", default=False,